The present invention relates generally to integrated biochemical sensors and especially relates to a miniature reference electrode of the liquid junction type and an integrated solid state electrochemical sensor making use of such a reference electrode.
Much of the recent literature on new electrochemical sensors has centered on the application or development of chemically sensitive semiconductor devices, with ISFET's (Ion Sensitive Field Effect Transistors) enjoying a predominate role. The solid state construction of these devices makes them attractive for biochemical sensing for several reasons: (1) They are batch fabricated in silicon, using planar integrated circuit technology, which reduces individual cost and variations in device characteristics. (2) They offer the possibility of "smart" multisensors, allowing for on-site signal processing. (3) They are rugged yet very small, which is an ideal combination for in vivo applications. Despite these advantageous characteristics, the development of commercially available sensors has been hampered by some very basic technological problems.
One of the problems that has restricted the use of ISFET's as a viable biomedical sensor has been the unavailablity of a suitable solid state reference electrode. A true solid state reference electrode would comprise a material whose interfacial potential remains invariant for different electrolytes and varying concentrations of aqueous electrolyte. A few attempts to discover such an electrochemical material have been proposed, (for instance in U.S. Pat. No. 4,269,682), with varying degrees of success. Since the interfacial potential is fundamentally limited by interfacial exchange currents, it appears unlikely that such a material will ever exist. It is precisely this difficulty that has forced the majority of investigators to rely on conventional reference electrodes and miniaturize them for compatibility with the ISFET. As expected, such miniaturization is subject to trade-offs. As an example, to miniaturize the conventional liquid junction reference electrode, smaller internal reference solution volumes are required. However, to prevent diffusional losses, which alter the reference potential, higher liquid junction impedances are necessitated. Also, fabrication of a conventional liquid junction type reference electrode is usually not process compatible with planar fabrication techniques and therefore not easily integrated with the ISFET. Thin film metal depositions are process compatible and several investigations have involved the use of a thin film AG/AgCl electrode which is either in direct contact with a test solution of constant Cl.sup.- concentration or the Ag/AgCl is covered with a polymer saturated with a solution of Cl.sup.-. However, both types are severely restricted by the variance of the reference electrode potential to changing concentration of its primary ion.
An object of the present invention is to provide a miniature reference electrode of the liquid junction type which can be batch produced using planar integrated circuit technology.
Another object of the invention is to provide a fully integrated electrochemical sensor comprising an Ion Sensitive Field Effect Transistor and a liquid junction reference electrode integrated on the same chip.